RMs and RM mixtures can be used to make optical films, like compensation, retardation or polarisation films, e.g. for use as components of optical or electrooptical devices like LC displays, through the process of in-situ polymerisation. The optical properties of the films can be controlled by many different factors, such as mixture formulation or substrate properties.
Chiral liquid crystal (CLC) films prepared from RMs are suitable for example for use as reflective polarisers or brightness enhancement films (BEF). The optical properties of the CLC film, like the maximum reflection wavelength and the bandwidth of the reflected wavelength band, can be controlled for example by changing the birefringence and/or the chirality of the RM material.
Prior art reports several ways of preparing a CLC film with a Bragg reflection peak that covers the visible spectrum. Such films are generally described as broadband CLC films. The prior art describes two basic methods of preparing a broadband CLC's, either by using band broadening techniques or by combining three or more narrow band CLC's.
As mentioned above, an important material property that can influence the bandwidth of a CLC is the birefringence of the LC material. LC materials with higher birefringence give wider bandwidth than LC materials with low birefringence. The relationship between bandwidth and birefringence for a CLC is given by the equation Δλ=Δn·p.
For example, KR2007-069512(A) and KR2006-43863A describe the preparation of multilayer CLC-BEFs. In these cases, three CLC films are used to make a broadband CLC film. JP 2000-281629 A, WO2007/142206 A1 and GB 2 395 201 A disclose high birefringence RM single compounds for use in wide band CLC films.
However, the materials and films described in prior art show several problems. For example, either a diffusion process is needed to make the film, in which case the processing time is too long, and the line speed too slow, or a multi-layer CLC approach is employed in which more than two layers are required, or the materials disclosed are considered unsuitable for mass production for either cost or technical reasons.
Therefore, there is still a need for materials and methods suitable for the preparation of CLC polymer films do not have the drawbacks as reported in prior art. In particular there is a need for CLC polymer films having a broader bandwidth than that achieved with RM materials and mixtures as disclosed in prior art.
This invention has the aim of providing improved RM mixtures for the preparation of CLC polymer films having the desired properties and not having the drawbacks of the prior art as explained above. Another aim of the invention is to extend the pool of RM mixtures and CLC polymer films available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing RM mixtures and CLC polymer films as claimed in the present invention.
This invention describes RM mixtures containing high birefringence singles that may be used to produce films that can be combined to give a broad band CLC. Furthermore, because the RM mixtures contain high birefringence components, only two films, or even only one film, are required to make a broad band film. Furthermore, this invention describes RM mixtures that can be used to make CLC polymer films that have a selective reflection peak in the visible, are suitable for mass production and are suitable for processing on industry standard coating machines. The RM mixtures can be coated to give well aligned optical films which may be combined with each other and with other optical films to produce a brightness enhancement film suitable for LCD's.